1. Field of the Invention
This invention relates to a hermetic via in a ceramic substrate, which may be used in implantable devices.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Various approaches are known for fabricating hermetically sealed electrical circuit housings suitable for extended operation in corrosive environments, e.g., in medical devices implanted in a patient's body. For such applications, a housing may be formed of biocompatible and electrochemically stable materials and typically may include a wall containing multiple hermetic electrical feedthroughs. A hermetic electrical feedthrough is comprised of electrically conductive material which extends through and is hermetically sealed in the wall material.
One known approach for forming feedthroughs uses platinum thick film vias through 92% or 96% aluminum oxide ceramic with significant glass content. This glass content is susceptible to hydroxide etching that may occur as an electrochemical reaction to an aqueous chloride environment such as is found in the human body. This will, over extended time, compromise the hermeticity and structural stability of the feedthrough. Typically, 92% aluminum oxide ceramic is used in conjunction with a platinum/glass or platinum/aluminum oxide thick film paste. These material systems are generally formulated to optimize coefficient of thermal expansion mismatches and achieve a hermetic feedthrough. However, use of metal/insulator frit significantly reduces the conductive volume of the feedthrough limiting the current carrying capacity of the feedthrough.
An alternative approach uses an assembled pin feedthrough consisting of a conductive pin that is bonded chemically at its perimeter through brazing or the use of oxides, and/or welded, and/or mechanically bonded through compression to a ceramic body. Typically, gold is used as a braze material that wets the feedthrough pin and the ceramic body resulting in a hermetic seal. Wetting to the ceramic body requires a deposited layer of metal such as titanium. This layer acts additionally as a diffusion barrier for the gold.
It is also known that tungsten, platinum or platinum-glass form vias in ceramics, such as alumina or zirconia. However, it has been difficult to form hermetic seals with the ceramic substrates, since platinum is a relatively soft material that shrinks from the walls of the via during sintering. The use of a glass frit with the platinum, for example, generally yields weak vias that do not bond well with the platinum. It is possible to obtain a hermetic via after firing, only to lose hermeticity during post-processing, such as soldering or brazing, due to the formation of worm holes in the via.
The shortcoming of using glass frit in the conductive materials is related to (1) the low electrical conductivity of the glass and (2) the weak bond between the via and the ceramic substrate after heat treatment. One alternative to glass frit binder involves using a metal oxide as the binder. Alternately the glass frit is replaced with an active metal as the bonding agent.
Alternative feedthrough approaches use a metal tube co-fired with a green ceramic sheet. The hermeticity of the metal/ceramic interface is achieved by a compression seal formed by material shrinkage when the assembly is fired and cooled. The use of a tube inherently limits the smallest possible feedthrough to the smallest available tubing. Acceptable results have been reported when using tubes having a diameter greater than 0.040 inches in ceramic substrates that are at least 0.070 inches thick.